Devices for optically determining a filling level have the advantage over mechanical and electrical devices which are wide-spread in use that they do not have any moving components and they are therefore not susceptible to wear and mechanical failure.
In this respect reference may be made to U.S. Pat. No. 3,424,004 disclosing a device for optically determining a filling level, including a light guide body which projects from above into the tank for containing a liquid and which has reflection faces arranged in respective pairs at different heights along the longitudinal extent of the vertically extending light guide body. The reflection faces are displaced relative to each other horizontally and can be observed through the upward end face of the light guide body. The reflection faces reflect light only when they are not immersed in the liquid in the tank. The level of liquid is then between the reflection faces which respectively appear light and dark.
Consideration may also be given to DE-A1-31 28 925 disclosing a level sensor for liquids, comprising a light guide body which dips into the liquid in a container and which is of a rectangular cross-section. The light guide body is of a stepped configuration at both narrow sides, the shoulders of the steps being formed by first and second reflection faces. Arranged in the upper end face of the light guide body are a light source and a light receiver. The light which is emitted downwardly from the light source into the light guide body is reflected at the first reflection faces which are not immersed in the liquid, totally in the direction of the second reflection faces and from those to the light receiver at the upward end of the light guide body. The signal outputted by the light receiver is greater or smaller, depending on the respective number of reflection faces which are operative to reflect the light, that is to say depending on the respective filling level, and that signal can thus serve to indicate the filling level.
Such a device can distinguish between discrete filling level heights, with the number of different filling levels that can be ascertained being predetermined by the number of first and second reflection faces respectively. In order to permit virtually continuous display of the filling level, it is possible to provide a large number of steps with first and second reflection faces respectively. In that case however the reflection faces become relatively small and as a result there is the danger of drops of liquid remaining clinging thereto when the level of the liquid in the container falls. That impedes total reflection and simulates a filling level which is higher than the actual level of liquid in the container.
Reference may also be made to DE-A1-32 43 839, more especially FIG. 8 thereof, disclosing a device in which a plurality of light guide bodies each having a respective reflection face are disposed at different heights in the side wall of a liquid container, the photosensors thereof being connected in succession. That arrangement is complicated and susceptible to trouble as leaks can occur at the locations where components are passed through the side wall of the structure.
DE-C-100 35 263 also discloses a device for optically determining the level of a liquid in a container, comprising a light guide body which in use projects into the container and which has an end face with an entry region for the light and an exit region for the light, together With a stepped configuration in respect of a side face, to afford reflection faces for selective reflection of light, depending on whether a reflection face is or is not immersed in the liquid. In that arrangement, provided in the light guide body between the narrow sides are gaps so that light which is reflected from a reflection face on to a second reflection face in opposite relationship therewith can only pass through ribs formed between the gaps. The production of such a light guide body is difficult however if it is to have a large number of reflection faces to provide a virtually continuous display of the filling level of liquid in the container.